The control of engine revolution rate in most recent construction machines, such as hydraulic shovels, is carried out by using a motor to drive a governor lever of an engine in response to an input signal which is obtained as a variation of the electric resistance of a potentiometer. In an operation method for this potentiometer, as illustrated in FIG. 3, a dial potentiometer 11 is rotated to the desired position to send a throttle signal to the governor controller 17 of the engine 16. The governor controller 17 transmits a drive signal to a governor motor 18 to drive a governor lever of the engine 16, and a feedback signal is transmitted from a sensor 21 associated with the governor motor 18 to the governor controller 17 so that the revolution rate of engine 16 is regulated in response to the received drive signal. Reference number 19a denotes a starter switch, while reference number 19b denotes a starter for the engine 16.
Lever potentiometers have been employed, as shown in FIG. 4, but a dial potentiometer is advantageous in that, as it does not require as much space on a control panel, the control panel can have a compact arrangement. However, the dial potentiometer has been employed primarily for single engine construction machines.
Most large-sized construction machines employ at least two drive engines because a single drive engine cannot provide sufficient power for all operations. While, in the case of the lever potentiometer, two levers 12 and 13 can be positioned, as shown in FIG. 4, for simultaneous operation, a large force is required for such simultaneous operation of both levers. Moreover, a large lever stroke requires a large space in the control panel, and the construction of the control panel is complex.
On the other hand, the dial potentiometer does not require a large force, it can be mounted in a compact space, and the construction of the control panel is simple. However, for a two-engine construction machine employing dial type potentiometers 14, 15, there is a problem in that both engines cannot be adjusted simultaneously unless both dials 14 and 15 are rotated simultaneously, thus requiring both hands of the operator, as shown in FIG. 5. Thus, the operability is unsatisfactory.